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Barn swallow

Barn swallow

The barn swallow (Hirundo rustica) is the most widespread species of swallow in the world.[2] It is a distinctive passerine bird with blue upperparts and a long, deeply forked tail. It is found in Europe, Asia, Africa and the Americas. In Anglophone Europe it is just called the swallow; in Northern Europe it is the only common species called a “swallow” rather than a “martin“.[3]

There are six subspecies of barn swallow, which breed across the Northern Hemisphere. Four are strongly migratory, and their wintering grounds cover much of the Southern Hemisphere as far south as central Argentina, the Cape Province of South Africa, and northern Australia. Its huge range means that the barn swallow is not endangered, although there may be local population declines due to specific threats.

The barn swallow is a bird of open country that normally uses man-made structures to breed and consequently has spread with human expansion. It builds a cup nest from mud pellets in barns or similar structures and feeds on insects caught in flight.[4] This species lives in close association with humans, and its insect-eating habits mean that it is tolerated by humans; this acceptance was reinforced in the past by superstitions regarding the bird and its nest. There are frequent cultural references to the barn swallow in literary and religious works due to both its living in close proximity to humans and its annual migration.[5] The barn swallow is the national bird of Austria and Estonia.

Description

Reported range from observations submitted to eBird shows the migration pattern of the species     Year-round range     Summer range     Winter range

The adult male barn swallow of the nominate subspecies H. r. rustica is 17–19 cm (6.7–7.5 in) long including 2–7 cm (0.79–2.76 in) of elongated outer tail feathers. It has a wingspan of 32–34.5 cm (12.6–13.6 in) and weighs 16–22 g (0.56–0.78 oz). It has steel blue upperparts and a rufous forehead, chin and throat, which are separated from the off-white underparts by a broad dark blue breast band. The outer tail feathers are elongated, giving the distinctive deeply forked “swallow tail”. There is a line of white spots across the outer end of the upper tail.[4] The female is similar in appearance to the male, but the tail streamers are shorter, the blue of the upperparts and breast band is less glossy, and the underparts paler. The juvenile is browner and has a paler rufous face and whiter underparts. It also lacks the long tail streamers of the adult.[2]

The song of the male barn swallow is a cheerful warble, often ending with su-seer with the second note higher than the first but falling in pitch. Calls include witt or witt-witt and a loud splee-plink when excited (or trying to chase intruders away from the nest).[4] The alarm calls include a sharp siflitt for predators like cats and a flitt-flitt for birds of prey like the hobby.[6] This species is fairly quiet on the wintering grounds.[7]

The distinctive combination of a red face and blue breast band render the adult barn swallow easy to distinguish from the African Hirundo species and from the welcome swallow (Hirundo neoxena) with which its range overlaps in Australasia.[2] In Africa the short tail streamers of the juvenile barn swallow invite confusion with juvenile red-chested swallow (Hirundo lucida), but the latter has a narrower breast band and more white in the tail.[8]

Taxonomy

The barn swallow was described by Carl Linnaeus in his 1758 10th edition of Systema Naturae as Hirundo rustica, characterised as “H. rectricibus, exceptis duabus intermediis, macula alba notatîs“.[9]Hirundo is the Latin word for “swallow”; rusticus means “of the country”.[10] This species is the only one of that genus to have a range extending into the Americas, with the majority of Hirundo species being native to Africa. This genus of blue-backed swallows is sometimes called the “barn swallows”.[2][3]

The Oxford English Dictionary dates the English common name “barn swallow” to 1851,[11] though an earlier instance of the collocation in an English-language context is in Gilbert White’s popular book The Natural History of Selborne, originally published in 1789:

The swallow, though called the chimney-swallow, by no means builds altogether in chimnies [sic], but often within barns and out-houses against the rafters … In Sweden she builds in barns, and is called ladusvala, the barn-swallow.[12]

This suggests that the English name may be a calque on the Swedish term.

There are few taxonomic problems within the genus, but the red-chested swallow—a resident of West Africa, the Congo basin, and Ethiopia—was formerly treated as a subspecies of barn swallow. The red-chested swallow is slightly smaller than its migratory relative, has a narrower blue breast-band, and (in the adult) has shorter tail streamers. In flight, it looks paler underneath than barn swallow.[8]

Subspecies

In slow motion

The preferred habitat of the barn swallow is open country with low vegetation, such as pasture, meadows and farmland, preferably with nearby water. This swallow avoids heavily wooded or precipitous areas and densely built-up locations. The presence of accessible open structures such as barns, stables, or culverts to provide nesting sites, and exposed locations such as wires, roof ridges or bare branches for perching, are also important in the bird’s selection of its breeding range.[4]

It breeds in the Northern Hemisphere from sea level to typically 2,700 m (8,900 ft),[27] but to 3,000 m (9,800 ft) in the Caucasus[4] and North America,[28] and it is absent only from deserts and the cold northernmost parts of the continents. Over much of its range, it avoids towns, and in Europe is replaced in urban areas by the house martin. However, in Honshū, Japan, the barn swallow is a more urban bird, with the red-rumped swallow (Cecropis daurica) replacing it as the rural species.[2]

In winter, the barn swallow is cosmopolitan in its choice of habitat, avoiding only dense forests and deserts.[29] It is most common in open, low vegetation habitats, such as savanna and ranch land, and in Venezuela, South Africa and Trinidad and Tobago it is described as being particularly attracted to burnt or harvested sugarcane fields and the waste from the cane.[7][30][31] In the absence of suitable roost sites, they may sometimes roost on wires where they are more exposed to predators.[32] Individual birds tend to return to the same wintering locality each year[33] and congregate from a large area to roost in reed beds.[30] These roosts can be extremely large; one in Nigeria had an estimated 1.5 million birds.[34] These roosts are thought to be a protection from predators, and the arrival of roosting birds is synchronised in order to overwhelm predators like African hobbies. The barn swallow has been recorded as breeding in the more temperate parts of its winter range, such as the mountains of Thailand and in central Argentina.[2][35]

Migration of barn swallows between Britain and South Africa was first established on 23 December 1912 when a bird that had been ringed by James Masefield at a nest in Staffordshire, was found in Natal.[36] As would be expected for a long-distance migrant, this bird has occurred as a vagrant to such distant areas as Hawaii, Bermuda, Greenland, Tristan da Cunha the Falkland Islands,[2] and even Antarctica.[37]

Feeding

Chicks in the nest

The barn swallow is similar in its habits to other aerial insectivores, including other swallow species and the unrelated swifts. It is not a particularly fast flier, with a speed estimated at about 11 metres per second (25 mph), up to 20 metres per second (45 mph) and a wing beat rate of approximately 5, up to 7–9 times each second.[38][39]

The barn swallow typically feeds in open areas[40] 7–8 m (23–26 ft) above shallow water or the ground often following animals, humans or farm machinery to catch disturbed insects, but it will occasionally pick prey items from the water surface, walls and plants.[4] In the breeding areas, large flies make up around 70% of the diet, with aphids also a significant component. However, in Europe, the barn swallow consumes fewer aphids than the house or sand martins.[4] On the wintering grounds, Hymenoptera, especially flying ants, are important food items.[2] When egg-laying barn swallows hunt in pairs, but otherwise will form often large flocks.[2]

The amount of food a clutch will get depends on the size of the clutch, with larger clutches getting more food on average. The timing of a clutch also determines the food given; later broods get food that is smaller in size compared to earlier broods. This is because larger insects are too far away from the nest to be profitable in terms of energy expenditure.[41]

Isotope studies have shown that wintering populations may utilise different feeding habitats, with British breeders feeding mostly over grassland, whereas Swiss birds utilised woodland more.[42] Another study showed that a single population breeding in Denmark actually wintered in two separate areas.[43]

The barn swallow drinks by skimming low over lakes or rivers and scooping up water with its open mouth.[28] This bird bathes in a similar fashion, dipping into the water for an instant while in flight.[33]

Swallows gather in communal roosts after breeding, sometimes thousands strong. Reed beds are regularly favoured, with the birds swirling en masse before swooping low over the reeds.[6] Reed beds are an important source of food prior to and whilst on migration; although the barn swallow is a diurnal migrant that can feed on the wing whilst it travels low over ground or water, the reed beds enable fat deposits to be established or replenished.[44]

Breeding

Four well grown chicks in a nest

H. r. rustica fledgling begging

Juvenile bird in Sussex

The male barn swallow returns to the breeding grounds before the females and selects a nest site, which is then advertised to females with a circling flight and song.[4] Plumage may be used to advertise: in some populations, like in the subspecies H. r. gutturalis, darker ventral plumage in males is associated with higher breeding success. In other populations,[45] the breeding success of the male is related to the length of the tail streamers, with longer streamers being more attractive to the female.[4][46] Males with longer tail feathers are generally longer-lived and more disease resistant, females thus gaining an indirect fitness benefit from this form of selection, since longer tail feathers indicate a genetically stronger individual which will produce offspring with enhanced vitality.[47] Males in northern Europe have longer tails than those further south; whereas in Spain the male’s tail streamers are only 5% longer than the female’s, in Finland the difference is 20%. In Denmark, the average male tail length increased by 9% between 1984 and 2004, but it is possible that climatic changes may lead in the future to shorter tails if summers become hot and dry.[48]

Males with long streamers also have larger white tail spots, and since feather-eating bird lice prefer white feathers, large white tail spots without parasite damage again demonstrate breeding quality; there is a positive association between spot size and the number of offspring produced each season.[49]

The breeding season of the barn swallow is variable; in the southern part of the range, the breeding season usually is from February or March to early to mid September, although some late second and third broods finish in October. In the northern part of the range, it usually starts late May to early June and ends the same time as the breeding season of the southernmost birds.[50]

Both sexes defend the nest, but the male is particularly aggressive and territorial.[2] Once established, pairs stay together to breed for life, but extra-pair copulation is common, making this species genetically polygamous, despite being socially monogamous.[51] Males guard females actively to avoid being cuckolded.[52] Males may use deceptive alarm calls to disrupt extrapair copulation attempts toward their mates.[53]

As its name implies, the barn swallow typically nests inside accessible buildings such as barns and stables, or under bridges and wharves.[54] Before man-made sites became common, it nested on cliff faces or in caves, but this is now rare.[2] The neat cup-shaped nest is placed on a beam or against a suitable vertical projection. It is constructed by both sexes, although more often by the female, with mud pellets collected in their beaks and lined with grasses, feathers, algae[54] or other soft materials.[2] The nest building ability of the male is also sexually selected; females will lay more eggs and at an earlier date with males who are better at nest construction, with the opposite being true with males that are not.[55] After building the nest, barn swallows may nest colonially where sufficient high-quality nest sites are available, and within a colony, each pair defends a territory around the nest which, for the European subspecies, is 4 to 8 m2 (43 to 86 sq ft) in size. Colony size tends to be larger in North America.[28]

In North America at least, barn swallows frequently engage in a mutualist relationship with ospreys. Barn swallows will build their nest below an osprey nest, receiving protection from other birds of prey that are repelled by the exclusively fish-eating ospreys. The ospreys are alerted to the presence of these predators by the alarm calls of the swallows.[28]

There are normally two broods, with the original nest being reused for the second brood and being repaired and reused in subsequent years. The female lays two to seven, but typically four or five, reddish-spotted white eggs.[2] The clutch size is influenced by latitude, with clutch sizes of northern populations being higher on average than southern populations.[56] The eggs are 20 mm × 14 mm (0.79 in × 0.55 in) in size, and weigh 1.9 g (0.067 oz), of which 5% is shell. In Europe, the female does almost all the incubation, but in North America the male may incubate up to 25% of the time. The incubation period is normally 14–19 days, with another 18–23 days before the altricial chicks fledge. The fledged young stay with, and are fed by, the parents for about a week after leaving the nest. Occasionally, first-year birds from the first brood will assist in feeding the second brood.[2] Compared to those from early broods, juvenile barn swallows from late broods have been found to migrate at a younger age, fuel less efficiently during migration and have lower return rates the following year.[57]

The barn swallow will mob intruders such as cats or accipiters that venture too close to their nest, often flying very close to the threat.[47] Adult barn swallows have few predators, but some are taken by accipiters, falcons, and owls. Brood parasitism by cowbirds in North America or cuckoos in Eurasia is rare.[4][28]

Hatching success is 90% and the fledging survival rate is 70–90%. Average mortality is 70–80% in the first year and 40–70% for the adult. Although the record age is more than 11 years, most survive less than four years.[2] Barn swallow nestlings have prominent red gapes, a feature shown to induce feeding by parent birds. An experiment in manipulating brood size and immune system showed the vividness of the gape was positively correlated with T-cell–mediated immunocompetence, and that larger brood size and injection with an antigen led to a less vivid gape.[58]

The barn swallow has been recorded as hybridising with the cliff swallow (Petrochelidon pyrrhonota) and the cave swallow (P. fulva) in North America, and the house martin (Delichon urbicum) in Eurasia, the cross with the latter being one of the most common passerine hybrids.[47]

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